Concentrations of progesterone, a metabolite of PGF2α, prolactin, and luteinizing hormone during development of idiopathic persistent corpus luteum in mares.

The study explores the behavior of certain hormones during the development of persistent corpus luteum (PCL) in mares. The research included three experiments, looking at several hormones and their concentrations at different times after ovulation. The findings indicate that luteinizing hormone (LH) and prolactin are lower in mares developing PCL immediately after ovulation, while progesterone and a metabolite of PGF2α, tend to decrease much slower in comparison to normal horses.

Explaining the Experiments and their Results

• Experiment 1: The researchers began by taking daily blood samples from a group of normal mares (n=5) and a group that developed PCL (n=5). Measurements were taken starting from the day of ovulation (Day 0) to Day 20. Analyses of these samples showed that in the immediate post-ovulation period, the mares developing PCL had significantly lower levels of LH and prolactin compared to normal mares. This demonstrated the reduction of these hormones early in the process of PCL development.
• Experiment 2: The second experiment involved the administration of a gonadotropin-releasing hormone antagonist to some animals (n=6). This significantly reduced LH concentration during Days 1 to 6 compared to controls (n=6). But the results did not support the hypothesis that low LH levels in the post-ovulatory period increase the frequency of PCL, suggesting that some other factor might be influencing the development of PCL.
• Experiment 3: The third experiment combined data from previous studies to examine larger group of mares looking at levels of P4, PGFM (a metabolite of PGF2α), and prolactin between Days 12 and 20. The researchers found that progesterone (P4) fell abruptly in normal mares from Day 13 onwards, while in PCL mares the P4 decrease was gradual and partial. Additionally, PCL mares had lower concentrations of the PGFM and prolactin on the day when decrease in P4 was most pronounced. Further analysis revealed no significant differences in the concentrations of PGFM and prolactin between mares with partial luteolysis and those without.

Conclusions Drawn from the Experiments

• Post-ovulatory LH and prolactin were found to be lower in the mares suffering from PCL.
• Attempt to reduce post-ovulatory LH through treatment didn’t increase the incidence of PCL, suggesting that PCL occurrence isn’t simply a result of low LH concentrations.
• Both PGFM and prolactin were discovered to be lower in PCL mares on the day with the most pronounced P4 decrease, pointing towards an altered hormonal profile in the development and persistence of PCL.